Field oxide layers electrically isolate semiconductor devices from one another. The most common technique for their formation is termed LOCOS isolation (for LOCal Oxidation of Silicon). Silicon dioxide (SiO.sub.2) is formed on silicon surfaces through a process termed oxidation. In the formation of field oxides, SiO.sub.2, is thermally grown to thicknesses of between 3,000 to 10,000 angstroms. Usually, oxidation is accomplished by exposing the silicon to an oxidant ambient, such as oxygen (O.sub.2) or water (H.sub.2 O), at elevated temperatures. Oxide is formed on those areas which are not covered by an oxidation mask, such as silicon nitride.
Silicon nitride is deposited by chemical vapor deposition (CVD), and photolithographically patterned to form the oxidation mask, using a dry etch. Silicon nitride is an effective mask due to the slow speed with which oxygen and water vapor diffuse in the nitride (typically only a few tens of nanometers of nitride are converted to SiO.sub.2 during the field oxide growth process). Therefore, the nitride layer thickness is selected according to the time needed for the field oxidation step. Typically, the nitride masking layer is deposited to a thickness of between 1,000 and 2,000 angstroms. After field oxidation, the masking layer is removed by a wet etch for subsequent device formation in the regions previously under the mask.
Field oxide layers function to prevent parasitic conditions between devices, such as punchthrough. Although punchthrough can be reduced by separating devices by adequate distances, field oxide layers are important in that they help to decrease this distance, by providing an electrical isolation layer. This is of great concern, especially in the manufacture of ultra large scale integrated (ULSI) circuits, where attempts are made to achieve semiconductor chips of maximum density mainly by scaling down line widths to make them narrower. Scaling down facilitates formation of leakage current paths, causing unwanted DC power dissipation, noise margin degradation, and voltage shift on a dynamic mode. Adequate electrical isolation between devices is necessary to prevent such problems.
Although field oxide layers presently provide many advantages in semiconductor technology, there are several problems created by their application to a ULSI circuit die, including "birds' beak" formation, as indicated in FIG. 1 at 110. Lateral oxidation from encroachment, or growth of oxide under the nitride edge 112, lifts the nitride layer 112 as it grows. This encroachment of the oxide between the nitride layer and the silicon substrate 114 forms a "birds' beak" structure 110, termed due to its slowly tapered shape. The effect from the "birds' beak" structure 110 is often called the narrow width effect. In order to minimize the "birds' beak" structure 110, a shorter oxidation time is required. When a short oxidation is used, the field oxide 116 thickness is reduced and the isolation properties for devices with small isolation are degraded. This effect limits the total number of devices that can be fit onto a single integrated circuit chip. A novel approach is required to apply the LOCOS process to ULSI circuits.
Another problem associated with the "birds' beak" structure is encountered during a later step of connecting metal to the source and drain regions of a MOS device. Any overetching during formation of the metal contact opening may expose the substrate regions under the source or drain region, shorting the device.
One approach to solving the problem of decreasing the "birds' beak" is to increase the H.sub.2 /O.sub.2 ratio during wet oxidation. Another approach is high pressure oxidation. Another prior art method employs the use of sidewall spacers to minimize the "birds' beak" phenomenon, such as shown in U.S. Pat. No. 5,393,693 to Ko et al.
Ko et al. utilize SiO.sub.2 spacers in the nitride layer opening in conjunction with several other processing techniques. The spacers act as lateral oxidation inhibitors. Oxygen implantation is employed as one of several complex steps taught by the '693 patent. Prior to oxygen implantation, the '693 patent requires etching a region of the silicon substrate to a depth of between 2,000 and 5,000 angstroms. In addition, oxidation is performed following deposition of a polysilicon layer, through which oxygen can diffuse, during oxidation in a non-oxidant, nitrogen ambient. The use of spacers does not lend itself well to reducing line widths, and is just one of several steps required to minimize the birds' beak effect.
There is a need for an effective method to minimize the "birds' beak" phenomenon, which results during growth of field oxide, utilizing fewer, less complicated steps and providing thermal cost saving steps. As devices get smaller, and more devices must be fit on each chip, any waste of precious chip space is very costly. Narrower line widths also reduce the density of oxidants available in openings in the nitride layer and hence make it difficult to obtain thick oxide growth. There is a need to minimize the encroachment of field oxide under nitride layers, to allow an increase in chip density. There is also a need to reduce the time required for thermal oxidation while still obtaining thick oxide growth.